By "coupler" are you talking about the trailer coupler that fits over the hitch ball and is attached to the tongue or A-frame?
My 4X8 utility trailer uses a 1 7/8, and my 6X12 utility trailer uses a 2 inch, and I purchased what is called a "convert a ball" kit. The ball shank stays on the hitch bar, and the two ball sizes are interchangable, just by pushing out the center pin. They make kits for 3 balls, and the third size is 2 5/16 inch. but they are costly because the shank has to be so large.